Process for the preparation of photographic emulsions



United States Patent 3,522,053 PROCESS FOR THE PREPARATION OF PHOTOGRAPHIC EMULSIONS Akira Kumai and Mamoru Tashiro, Minami-Ashigara Machi, Kanagawa, Japan, assignors to Fuji Shashin Film Kabushiki Kaisha, Minami-Ashigara Machi, Kana gawa, Japan N0 Drawing. Filed Feb. 4, 1966, Ser. No. 525,106 Int. Cl. G03c 1/02 US. Cl. 96114.7 9 Claims ABSTRACT OF THE DISCLOSURE Method for making silver halide photographic emulsions, particularly flocculation of silver halide dispersions in aqueous gelatin solutions by adding phenol and carefully regulating the pH value.

The present invention relates generally to photographic silver halide emulsions and more particularly to a process for preparing such emulsions.

Previously, photographic silver halide emulsions have been prepared by precipitating silver halide in aqueous gelatin solution, chilling and solidifying the emulsion into gel, then shredding and washing with cold water to remove undesirable by-product soluble salts.

As a result of the slow diffusion speed of the salts, the washing step usually requires considerable time, and the volume of gel so formed tends to increase to a large or small degree depending upon the properties of gelatin employed. Often, an undesired lowering of viscosity occurs.

An improved process for producing a photographic emulsion with a high silver halide content has been proposed which comprises making a dispersion of silver halide microcrystals in aqueous gelatin solution, coagulating by adding flocculating agent, and decanting separated supernatant liquid so as to remove unnecessary salts, Washing the flocculate with water after which a further amount of aqueous gelatin solution is added, and in most cases re-dispersing the flocculate by elevating the pH value to a predetermined value.

The aforementioned process can eliminate shredding, water-washing and the like which occupy a considerable time. In addition, the emulsion character can be varied greatly and improvements in photographic properties obtained. For instance, this process can realize an increased silver halide to gelatin ratio in the emulsion, which could not be increased in the prior process beyond a certain limit since the photographic emulsion must have a minimum gelatin concentration of about 34% in order to complete the gelling of the gelatin. For such reasons, the production of photographic emulsion according to this flocculation process has become increasingly important of recent. However, the flocculating process dilfers, one from another, especially by the nature of the flocculant used.

The oldest method used for flocculating gelatino-silver halide emulsions involves the addition to the emulsion of salts such as sodium sulphate. Gelatin is thus flocculated out accompanying silver halide. This method, however, is disadvantageous in that a very large quantity of the salt is necessary to effect a complete flocculation. Moreover, the precipitate thus formed is so rapidly dissolved again 3,522,053 Patented July 28, 1970 in water that it must be handled with great care when washed out with water.

There has also been proposed in which an organic solvent, miscible with water but incapable of dissolving gelatin, is used for flocculating the emulsions. This process,; in turn, has proved disadvantageous because of the great consumption of the organic solvents and the fact that various salts which are insoluble in the solvent mixture are also precipitated. Moreover, the precipitate thus obtained cannot be washed out with water.

Another method of flocculating emulsions consists in chemical modification of gelatin. In this method, for example, gelatin is first caused to react with phthalic acid anhydride and thereafter used for the production of photographic emulsion. This type of gelatin derivative can be precipitated in an aqueous solution at a pH value of around 3. However, this method is accompanied by the difliculty that a portion of the gelatin decomposes because of elevated pH value (in the region of 10) which is usually required for carrying out the reaction to produce the gelatin derivative. Moreover, it is usually diflicult to prepare a photographic emulsion having high sensitivity by using this type of gelatin derivatives.

Another class of proposed flocculants are those capable of, forming a complex with gelatin which under reduced pH becomes insoluble and precipitates.

Examples of such compounds hitherto proposed include alkyl sulphonic acids having long carbon chains (C -C The disadvantages of this process stem from the fact that flocculation only takes place at extremely lowered pH range (pH 4), and that the photographic emulsion prepared according to this procedure cannot be coated uniformly onto a support.

High molecular acid-insoluble compounds containing carboxyl groups have also been proposed as flocculants, but the use of such flocculants causes excessive increase in viscosity resulting in poor redispersability and excessive swelling of the coated layer.

Accordingly, an object of the present invention is to provide novel photographic gelatino-silver halide emuls1ons.

Another object of the present invention is to provide novel methods for the fabrication of gelatino-silver halide photographic emulsions.

A further object of the present invention is to prepare certain specified dispersions of silver halides useful for the preparation of gelatino-silver halide photographic emulsions.

Still a further object of the present invention is to prepare certain specified dispersions of silver halides from which water soluble by-products may be readily removed.

Still another object of the present invention is to provide a method of washing specified silver halide dispersions in which substantially all of the liquid may be removed, thus permitting substantially complete removal of the water-soluble impurities therein.

Other objects of the present invention will become obvious from the specification.

The present invention is based upon the discovery of a novel flocculating agent which is entirely dilferent from any known type enabling easy and rapid removal of undesirable soluble salts from gelatino-silver halide emulsion, also enabling easy re-dispersion of the resultant flocculate. Moreover, it enables the production of 'a photographic emulsion having excellent photographic characteristics such as sensitivity, fog, etc.

More particularly, the process of the present invention comprises intermixing aqueous solution of a water-soluble silver salt with aqueous solution of a water-soluble halide in the presence of gelatin so as to prepare a dispersion of fine silver halide crystal, then adding thereto at least one of the aromatic phenols or their derivatives which will be hereinbelow defined and adjusting the pH value to cause precipitation, decanting supernatant liquid to remove soluble salt formed as by-product, washing the resultant precipitate with Water, thereafter further incorporating therewith, if desired, gelatin and other various types of sensitizers such as labile sulfur compounds, aurous compounds and the like well known in the art, after which heating under stirring to effect re-dispersion and subjecting tochemical sensitization to give a gelatinosilver halide photographic emulsion in a manner more facile and rapid than any of the known processes.

The phenolic compounds employed in the present invention are represented by the following general formula:

wherein R is selected from the group consisting of hydrogen, an alkyl group containing 1 to 5 carbon atoms and OH group, and R is selected from the group consisting of hydrogen, --O-H and COOH groups.

Examples of such compounds include: phenol, catechol, phloroglucinol, salicyclic acid, cresol, thymol, p-oxybenzoic acid, 3,5-dioxybenzoic acid, p-tert.-butylphenol, etc.

The compounds according to the invention are particularly advantageous over prior known flocculating agents because they are readily available much more inexpensively and in uniform quality as commercial chemicals having simple chemical structure than other known types of fiocculating agents, and because the re-dispersability of the precipitate compound of gelatin, silver halide microcrystals, flocculating agents, etc. can be conducted in such an easy way as being incomparable to prior processes, and further because the compounds used in the present invention have no deteriorative effects upon the photographic emulsion layers obtained therewith. In addition, no particular antiseptic treatment of the photo graphic emulsion is needed since such phenols generally possess sufiicient antiseptic activity to protect photographic gelatino-silver halide emulsion during cold storage thereof.

The flocculating compounds of the present invention may be added directly to the emulsion when they are liqquid, or they may preferably be added in the form of solutions in suitable solvents which 'have no harmful eifects upon the emulsion, for example, water or methanol, in order to cause uniform distribution of the flocculants in the emulsion.

The particular quantity of the phenolic compounds used in a given emulsion usually lies within the range of -300% by weight based on the gelatin in the emulsion at the time of flocculation. Although it has been found that the addition of -100% by weight of gelatin is quite adequate to accomplish the desired flocculation, these limits are only illustrative. Addition of less than 10% will cause no appreciable flocculation. On the other hand, addition of as much as more than 300% will give rise to an irreversible coagulation of gelatin.

The fiocculants of the present invention may preferably be used in a combination of two or more phenolic compounds, especially in view of improving flocculability or redispersability, etc. 'They may be suitably used in a combination in which at least one has a relatively high acidic character, to eliminate further addition of acid for the adjustment of the pH value. Since many compounds represented by the foregoing general formula are usually acidic, the pH value of the resultant emulsion will generally decrease to below 5. Additional acids such as acetic acid and sulfuric acid may be incorporated to facilitate flocculation, especially when an emulsion having relatively high pH, such as ammoniacal emulsion, is used. The pH is suitably maintained during flocculation within the range of 3.5 to 5.5 since re-dispersion becomes difficult at the pH below 3.

When preparing a photographic emulsion in accordance with the process of the present invention, a portion or whole of gelatin added for redispersion may be replaced with a 'hydrophilic film-forming polymer such as polyvinyl alcohol, hydroxyethyl cellulose, etc., all well known in the art.

The present invention will be illustrated in greater detail in conjunction with the following specific examples which set out representative preparations of the novel emulsions of this invention, which, however, are not limited to the details therein set forth which are intended to be illustrative only.

EXAMPLE 1 A solution of 105 g. of silver nitrate in 975 g. of distilled water was added under stirring in the course of one minute to an aqueous solution containing 15 g. of gelatin, g. of potassium bromide and 25 g. of potassium iodide dissolved in 450 g. of water. During mixing, the solution was kept at 50 C. The resulting emulsion was digested at 50 C. for 15 minutes and then 30 ml. of a 75% aqueous phenol solution and 14 ml. of a 10% methanolic solution of salicyclic acid added thereto. Silver halide microcrystals rapidly fiocculated at the bottom of the kettle together with gelatin. After decanting clear supernatant liquid, the fiocculate was repeatedly washed with water containing 0.2% salicyclic acid. Then it was added with 65 g. of active gelatin, 18 ml. of a 2 N solution of sodium carbonate, g. of distilled water and a mixture of 20 ml. of 0.01% chloroauric acid with 10 ml. of a 1% potassium thiocyanate and heated at 50 C. under stirring to cause re-dispersion and chemical sensitization. The gelatino-silver iodo bromide photographic emulsion thus obtained was highly sensitive with less fogging.

EXAMPLE 2 To 900 g. of distilled water was added 100 g. of silver nitrate, and the resulting solution added into a solution containing 10 g. of inert gelatin, 82 g. of potassium bro mide and 5 g. of potassium iodide dissolved in 700 g. of water over a period of 30' minutes, while being thoroughly stirred. The mixture was kept at 60 C. during mixing. After digesting the resultant mixture for 15 minutes at 60 C., ml. of a 10% aqueous solution of 3,5-dihydroxybenzoic acid was added to immediately flocculate out the emulsion. After decanting clear supernatant liquid, the flocculate was washed two times each with 500 ml. of a 0.2% aqueous acetic acid and thereafter heated at 50 C. while stirring after adding 200 g. of active gela tin, 20 ml. of 2 N aqueous sodium carbonate solution, 1,000 g. of distilled water and a mixture of 10 ml. of a 0.01% chloroauric acid with 5 ml. of a 1% aqueous ammonium thiocyanate. The gelatino-silver iodobromide photographic emulsion thus obtained did not differ from a high speed, fogless negative emulsion prepared in the conventional way.

EXAMPLE 3 A solution of 100 g. of silver nitrate in 550 g. of distilled water was poured, during stirring, over a period of one minute at 50 C., into a solution containing 45 g. of sodium chloride and 12 g. of inert gelatin dissolved in 1,200 g. of distilled water. After the mixture was stirred at 50 C. for an additional 5 minutes, 60 ml. of a 10% methanolic solution of p-t-butyl phenol and 25 ml. of a 10% methanolic solution of p-cresol were added to flocculate out silver chloride together with gelatin. After decanting the supernatant liquid, g. of active gelatin, 600 g. of distilled water and 70 ml. of a 2 N sodium carbonate were added and stirred for 35 minutes at 40 C. There was then added 75 g. of inert gelatin during stirring for further 35 minutes to completely dissolve the gelatin. The gelatino-silver chloride photographic emulsion thus prepared was especially suited for use in making photographic printing papers.

EXAMPLE 4 A full ammoniacal silver iodobromide photographic emulsion was prepared by mixing the following solutions.

Solution A:

Distilled water, ml. 1200 Silver nitrate, g. 300 Ammonia (sp. gr.=0.91), ml. 300 Solution B:

Distilled Water, ml. 700 Potassium bromide, g 300 Potassium iodide, g. 6 Gelatin, g. 70

The solution A was added under sufficient stirring into solution B over a period of 30 minutes While maintaining the temperature at 45 C. After cooling the mixture to 30 C., 150 ml. of a 10% aqueous 3,5-dihydroxybenzoic acid, 160 ml. of a 5% aqueous phenol and 640 ml. of a 10% sulfuric acid were added to fiocculate out the emulsion containing silver iodobromide microcrystals. After decanting supernatant liquid, the precipitate was washed several times in cold water. After adding 1,200 ml. of distilled water thereto, 40% aqueous sodium hydroxide solution was added until a pH value of 6.6 was reached while keeping the temperature at 40 C. while continuing stirring, thereby efiecting re-dispersion. Then the temperature was raised to 50 C. and it was chemically ripened for 40 minutes after adding 7 ml. of a 0.01% methanolic solution of triethyl thiourea, 25 ml. of a 0.01% chloroauric acid and 20 ml. of a 0.1 N aqueous ammonium thiocyanate solution. The silver iodobromide emulsion obtained in this procedure was especially suited for use as medical X-ray film emulsion with adequate sensitivitygamma, and less fog.

Manifestly, the present invention is not limited to the detailed description presented above, but includes all modifications falling within the scope of invention, as defined in the sub-joined claims.

We claim:

1. A process for the production of photographic gelatino silver halide emulsions which comprises:

forming a silver halide dispersion which consists essentially of an aqueous solution of a water-soluble silver salt and a water-soluble halide with gelatin, flocculating said silver halide dispersion by adding at least one phenolic compound represented by the following general formula:

wherein [R is a member selected from the group consisting of a hydrogen atom, an alkyl group having 1 to 5 carbon atoms and a hydroxyl group, and R is a member selected from the group consisting of a hydrogen atom, a carboxyl group and a hydroxyl group, at a pH between 3.5 and 5.5,

said pH value being adjusted to between 3.5 and 5.5 by adding a water-soluble acid selected from the group consisting of a mineral acid and an organic acid to said dispersion to thereby flocculate said silver halide dispersion as a coagulum,

separating said coagulum from the supernatant liquid,

and

washing said coagulum with water and redispersing it at a pH value of at least 6.0.

2. The process according to claim 1, wherein said phenolic compounds are added in concentrations of 10 to 300% based on the dry weight of the gelatine.

3. The process according to claim 1, wherein said phenolic compounds are selected from the group consisting of phenol, p-cresol, m-cresol, o-cresol, resorcinol, catechol, p-oxybenzoic acid, 3,5-dihydroxy-benzoic acid, 3-methyl-6-isopropyl phenol, salicylic acid, phloroglucinol and ptert.-butylphenol.

4. The process according to claim 1, wherein said phenolic compounds are phenol and salicylic acid.

5. The process according to claim 1, wherein said phenolic compound is 3,5-dihydroxybenzoic acid.

6. The process according to claim 1, wherein said phenolic compounds are 3,5-dihydroxy-benzoic acid and phenol.

7. The process according to claim 1, wherein said phenolic compounds are 3,5-dihydroxybenzoic acid and salicylic acid.

8. The process according to claim 1, wherein said phenolic compounds are 3,5-dihydroxybenzoic acid and p-cresol.

9. The process according to claim 1, wherein said phenolic compounds are salicylic acid and p-cresol.

References Cited UNITED STATES PATENTS 3,218,169 11/1965 Kelly et al. 9694 3,241,969 3/1966 Hart et al. 9694 3,366,482 1/1968 Schaller et al. 9694 NORMAN G. TORCHIN, Primary Examiner M. F KELLEY, Assistant examiner US. Cl. X.R. 9694 

